Diffusion transfer photographic processes are well known in the art. Such processes have in common the feature that the final image is a function of the formation of an imagewise distribution of an image-providing material and the diffusion transfer of the imagewise distribution to an image-receiving layer. In general, a diffusion transfer image is obtained first by exposing to actinic radiation a photosensitive element, or negative film component, which comprises at least one light-sensitive silver halide layer, to form a developable image. Thereafter, this image is developed by applying an aqueous alkaline processing fluid to form an imagewise distribution of soluble and diffusible image dye-providing material, and transferring this imagewise distribution by diffusion to a superposed image-receiving layer of an image-receiving element, or positive film component, to impart a transfer image thereto.
The aqueous processing compositions employed in diffusion transfer processes are usually highly alkaline (e.g., pH&gt;12). After processing has been allowed to proceed for a predetermined period of time, it is desirable to neutralize the alkali of the processing composition to prevent further development and image dye transfer, and, in some instances, subsequent oxidation which may have a material and substantial effect upon the stability to light of the resulting image in the image-receiving layer.
Accordingly, a neutralizing layer, typically a nondiffusible acid-reacting reagent, is employed in the film unit to lower the pH from a first (high) pH of the processing composition to a predetermined second (lower) pH. For example, a polymeric acid neutralizing layer can be used such as disclosed in U.S. Pat. Nos. 3,362,819 and 3,415,644. Generally, the polymeric acid comprises a polymer containing acid groups, typically carboxy groups, which are capable of forming salts with alkali metals such as sodium or potassium which are usually included in the processing composition. In order to ensure that the pH reduction occurs after a sufficient, predetermined period and not prematurely so as to interfere with the development process, a timing layer is typically positioned before the neutralization layer.
Timing layers have been designed to operate in a number of ways including: (1) as a sieve which slowly meters the flow of alkali therethrough to the polymeric acid neutralizing layer as described in aforementioned U.S. Pat. No. 3,362,819 and in U.S. Pat. No. 3,421,893 ("sieve-type") and (2) as an alkali-impermeable barrier for a predetermined time interval before converting in a rapid and quantitatively substantial fashion to a relatively alkali-permeable condition, upon the occurrence of a predetermined chemical reaction, e.g., hydrolysis and beta-elimination, under basic conditions and known in the art as "hold and release," as disclosed in U.S. Pat. Nos. 3,575,701; 4,201,587; 4,288,523; 4,297,431; 4,391,895; 4,426,481; 4,458,001; 4,461,824 and 4,457,451.
Generally, an additional factor to be considered with regard to designing a timing layer possessing optimum alkali-permeability characteristics within the temperature range of optimum transfer processing is that the rate of the development process involved in diffusion transfer photography is temperature-dependent, i.e., at reduced temperatures, the development process becomes markedly slower; at higher temperatures, the rate of development is increased.
Accordingly, such a range of development rates imposes additional performance demands on the timing layer. More specifically, if a timing layer were to permit penetration by alkali to the neutralizing layer while development were still incomplete because of a low temperature slow-down of the development process, development shut-down would be premature and image formation would be incomplete. Similarly, at increased development rates resulting from the effects of higher temperatures, late release by a timing layer could cause over-development, producing images of reduced dye density. Therefore, to avoid the side effects of temperature variations, the timing layers have been typically designed to offer a temperature response substantially parallel to that of the development process, i.e., the permeability to alkali is directly dependent upon temperature.
However, as disclosed in aforementioned U.S. Pat. No. 3,421,893, "sieve-type" timing layers may also comprise materials exhibiting permeability to alkali which is inversely dependent upon temperature, i.e., temperature-inverting polyvinyl amides which exhibit superior cold temperature processing. More particularly, aforementioned U.S. Pat. No. 3,421,893 describes a timing layer which as a whole exhibits temperature inverting properties which enable it to assert a better measure of control over the polymeric acid neutralizing layer at cold temperatures, e.g., 10.degree. C., than would a non-temperature inverting timing layer which typically shows decreased permeability as the temperature is reduced to, e.g., 10.degree. C. As stated earlier, in this situation, the use of a non-temperature inverting timing layer could result in the maintenance of the transfer processing environment's high pH for such an extended time interval as to facilitate formation of transfer image stain and its resultant degradation of the positive transfer image's color definition.
Benefits are derived from using a temperature-inverting material in a process, e.g., the development of an exposed photosensitive photographic film unit, which depends upon permeation of liquids at a variety of temperatures. For example, as the ambient temperature decreases, the temperature inverting, e.g., polymeric, material of the timing layer tends to form hydrates and swells, thus facilitating permeation as function of the degree of swell of the polymer--deswelling being inherent with an increase in temperature. Further, it is well known that the diffusion rate of a liquid, e.g., an alkali, will increase as the temperature increases and that, in a typical diffusion transfer photographic process this rate is directly proportional to the progress of the transfer image formation per unit time. Hence, the benefit of devising a mechanism for controlling the diffusion rate inversely with temperature is recognized. Moreover, the desired result is to have the temperature inverting material approximately counteract changes in the diffusion rate of the permeating material with changes in temperature. Temperature inversion is, therefore, relative, since the precise properties desired would be dependent upon the response of the whole, e.g., photographic system, to changes in temperature.
Furthermore, extreme inverse temperature characteristics are generally not particularly desirable since the development of the photosensitive element of the system and the dye transfer are temperature dependent processes and should be functionally compatible with the temperature-permeation properties of the image-receiving element. Therefore, an ideal timing layer should provide the system which it comprises with the proper dye permeation-temperature properties so that the dye(s) may diffuse from the photosensitive element to the image-receiving element as a function of development to form a positive image in the image-receiving layer within a predetermined time, irrespective of the processing temperature employed.
It is thought by those of ordinary skill in the art that the temperature inverting properties possessed by certain materials may be attributable to the presence of a predetermined balance of hydrophobic groups to hydrophilic groups in the, e.g., polymer molecule. It is also thought that a probable mechanism through which temperature inversion occurs is by the formation of hydrogen bonds between the hydrophilic portion of the, e.g., polymer, and the hydrogen of the solvent at low temperatures; the hydrogen bonding being discouraged as the temperature of the material is raised due to thermal destruction. The system thereupon takes the form of a less hydrated, less-swollen, therefore, less-permeable, e.g., polymer, as a function of the increase in temperature. However, it is important to remember that the precise temperature inverting properties exhibited by the system are most likely a reflection of the response of the entire system to changes in temperature as opposed to the result of one particular component.
Depending upon the nature of materials desirably controlled through the utilization of a timing layer and the desired functional mode of the timing layer, the nature and permeability characteristics of a timing layer and the monomeric or polymerizable monomeric compounds thereof can be varied to suit particular applications. For instance, as stated earlier, a timing layer adapted to prevent the passage, or effect a "hold," of alkali for a predetermined period until the occurrence of a predetermined chemical reaction can assist in the control of environmental pH conditions in a photographic film unit. However, as is understood in the art, the presence in a timing layer of a polymer or other materials which adversely affect or negate the desired permeability properties of a timing layer is to be avoided.
As stated earlier, it is well known in the art that the development process generally becomes markedly faster at higher temperatures. Typically, timing layers have been designed so as to offer temperature responses substantially parallel to that of the development process, i.e., at elevated temperatures, the development process becomes markedly faster and the permeability of the timing layer to alkali is increased in order to minimize the side effects of temperature variations including premature or late interaction between the various components of the photographic system. However, as aforementioned U.S. Pat. No. 3,421,893 points out, at relatively high transfer processing temperatures, i.e., above approximately 27.degree. C., a premature decrease in the pH of the transfer processing composition occurs due, at least in part, to the rapid diffusion of alkali from the dye transfer environment and its subsequent neutralization upon contact with the polymeric acid layer.
Therefore, while such timing layers have been found to provide advantageous results as are described in the above-mentioned patents; nevertheless, their performance in some photographic systems is not completely satisfactory, e.g., where it is desirable to develop photographic systems at hot temperatures, e.g., above approximately 27.degree. C., there exists a need for "sieve-type" timing layers which exhibit temperature inverting properties, i.e., at higher temperatures, the development process becomes markedly faster and the permeability of the timing layer is decreased, to better control dye transfer resulting in desirable dye density.
Accordingly, as the state of the art for photographic systems advances, novel techniques and materials continue to be developed by those skilled in the art in order to attain the performance criteria required of such materials. There will always be a need for new timing layers that have advantages over those already known to the art; hence, investigations continue to be pursued to provide such advantages.
It has now been unexpectedly discovered that if a layer comprising a polyester urethane polymer(s) which is inert to alkali, i.e., does not become water-permeable, e.g., not permeable to an aqueous alkaline processing composition, in and of itself under typical film processing conditions, the layer exhibits permeability to alkali inversely dependent upon temperature and superior hot temperature processing performance, e.g., processing at a temperature above approximately 27.degree. C., is achieved, as evidenced by higher transfer image maximum densities and the elimination of cracking in the finished photograph.
Accordingly, the present invention relates to a novel diffusion transfer film unit which includes a layer, e.g., a timing layer or a diffusion control interlayer, which exhibits permeability to alkali inversely dependent upon temperature, and specifically, to a layer comprising polyester urethane polymers which are inert to alkali unexpectedly resulting in superior hot temperature processing.